Ketones palladium mediated

Unfortunately, it quickly became apparent that a shortfall in this proposal was an inability to prepare the desired vinyl halide 25 in a straightforward and selective manner [19]. In contrast, we reasoned that the selective formation of an enol sulfonate, such as the enol triflate 26a, could be controlled by judicious tuning of enolization conditions starting from the corresponding ketone, and that such an enol sulfonate would possibly be a substrate for a palladium-mediated coupling (Scheme 9.17). In this way a common intermediate from the previously defined synthesis, that is, the racemic ketone rac-13 or its cyano equivalent rac-5 could be used to generate the required enamide. 

Several examples of transition metal catalysis for the synthesis of piperidines appeared this year. Palladium catalyzed intramolecular urethane cyclization onto an unactivated allylic alcohol was described as the key step in the stereoselective synthesis of the azasugar 1-deoxymannojirimycin . A new synthetic entry into the 2-azabicyclo[3.3.1]nonane framework was accomplished through a palladium mediated intramolecular coupling of amine tethered vinyl halides and ketone enolates in moderate yields . A palladium catalyzed decarboxylative carbonylation of 5-vinyl... 

Hydroboration and oxidation of 160 yields an alcohol that is subsequently oxidized with PDC to give ketone compound 161. Enolization and triflation converts this compound to enol triflate 162, which can be further converted to x,/i-unsaturated ester 163 upon palladium-mediated carbonylation methox-ylation. The desired alcohol 164 can then be readily prepared from 163 via DIBAL reduction. Scheme 7 50 shows these conversions. 

Scheme8.2. Palladium-mediated Barbier-type cyclization of a bromoaryl ketone [37],...

An intramolecnlar palladium-mediated allylic alkylation via a ketone enolate of piperidinone 54 was reported by Williams and coworkers for the synthesis of R)-l-hydroxyquinine 57 (eqnations 17 and 18) °. The key step involves a palladium-mediated Sjv2 -type cyclization reaction of enol ether 55 in the presence of BnsSnF, giving rise to a quinuclidine ketone, which was immediately rednced to 56 to avoid equilibration and /3-elimination. Interestingly, none of the undesired C3-vinyl stereoisomer was observed. 

In a total synthesis of (+)-brefeldin A, the reagent was elaborated into the a,(3-enone shown, which participated in a palladium-mediated cyclopentene-forming reaction to afford the cxo-olefin. Stereoselectivity in the ring-forming reaction was 3.5 1 in favor of the desired isomer over the alternative trans-cyclopentene. Ozonolysis and reduction of the resulting ketone, followed by protection afforded the MEM ether shown, where all the relevant stereocenters of the final target were established (eq 18). ... 

Palladium-mediated syntheses of aldehydes and ketones from thiol esters (many examples of syntheses of A-heterocycles and natural compounds containing heterocyclic fragments) 04ACA87. 

Retrosynthetically, a base-catalyzed dimerization of 127 would afford stephacidin B. Avrainvillamide (127) was simplified as vinyl iodide 128 by cleavage of the dihydropyrano [2,3-g]indole-l-oxide moiety and palladium mediated coupling. An aminoacyl radical addition from 129 gave access to 128, while 129 could be derived from cyanide 130 through a hemiaminal formation/dehydration and conjugate addition. Finally, Strecker-like reaction of ketone 131 would fulfill nitrile 130 [55] (Scheme 22). 

In most of the palladium-mediated reactions, aryl halides have been used although several other types of electrophiles or alkenes may be employed. Aryl triflates give somewhat lower radiochemical yields and require the presence of lithium bromide. Diaryliodonium salts (e.g., diphenyliodinium bromide) have shown to be an interesting alternative in the synthesis of C-labeled ketones (Al-Qahtani and Pike 2000). 

When the reaction conditions were applied to allylic carbonate 8, the palladium-mediated cyclization occurred to form the 11-membered ketone 9 in 86% yield (eq 30). When the palladiun-catalyzed allylation of propargyl malonate 10 in the presence of BSA was combined with the rhodium-catalyzed Pauson-Khand t)fpe reaction, the tandem action of the two catalysts gave an excellent yield (92%) of bicyclopentenone 11 in one-pot (eq 31). ... 

This result is achieved with the methylation product 290. Palladium-mediated Sjj -substitution gives rise to the synthetically very attractive ketone 291. 

Scheme 5.47 Enantioselective palladium-catalyzed enolate arylation of racemic aminomethylene ketones 143 mediated by chiral ligands 144.

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